The invention relates to a method for operating a cross-flow filtration installation for a product, comprising at least one filtration module with product flow-through and permeate outflow, a pipeline for supplying the product to and removing it from the filtration module, a pump with an electric motor in the pipeline for supplying the product to the filtration module, as well as a device for setting the conveying flow of the pump. The invention also relates to a cross-flow filtration installation for executing this method.
Known cross-flow filtration installations operate in the operating modes of ultra-filtration, micro-filtration, nano-filtration and reverse osmosis. For all operating modes at least one pump is required for transporting a product to be filtered past the surface of filtration diaphragms. In the process a static pressure and a flow speed of the product occur at every filtration diaphragm, which cause a portion of the product to penetrate the filtration diaphragms as permeate.
A known optimization goal for such installations is a large filtration yield as a permeate quantity in liters per diaphragm surface in square meters and filtration time in hours. It is a further goal to achieve a high maximum degree of thickening of the remaining portion of the product as the retentate. The degree of thickening is determined in a centrifuge test as a portion of the wet sludge in percent of the retentate amount. Besides many other operating parameters, these goals are a direct function of the mentioned parameters of static pressure and flow speed of the product.
The mentioned optimization goals can only be achieved in a limited way, because in the operation of ultra-filtration, for example, known filtration diaphragms on an organic basis have a low permissible operating pressure of approximately 6 bar. Thus, in view of the filtration yield and the concentration of solids, there is the goal of always fully utilizing at least the preset permissible operating pressure. If therefore it is possible with a predetermined diaphragm size and predetermined product throughput per hour to operate at maximum permissible operating pressure, maximum concentration of the solids and viscosity of the retentate are simultaneously achieved.
In connection with diafiltration, high concentrations of solids at the end of the filtration also result in advantages during the subsequent washing out of the retentate. The amount of time and washing agent (water) depends heavily on this concentration of solids.
The desired operation at maximally permissible operating pressure and high solids concentration in the retentate now leads to problems, in particular when using non-volumetrically conveying pumps for the product. The relationship between the amount conveyed and the conveying pressure of such pumps changes with changes in the retentate properties. The influences of the following properties have an effect here:
the viscosity of the retentate,
the specific mass of the retentate, and
the intrinsically viscous or thixotropic behavior of the retentate.
If it is attempted to maintain the maximally permissible operating pressure at the diaphragm constant by regulating the filtration, large fluctuations of the product yield in liters per hour occur. If it is attempted to control these fluctuations in the yield at the pump, changes of the specific mass of the product in particular lead to changes of the power consumption of the drive motor of the pump in kilowatts and therefore to overloads with emergency stops of the delivery and therefore of the installation. Because of the intrinsically viscous behavior, such interruptions of the delivery at high solids concentration of the retentate, however, have the result that the installation then can no longer be restarted. Often, damage to the filtration modules occurs, and the jammed-up product can only be removed by manual cleaning and with a large expenditure of time.
In connection with known large and small installations, the mentioned circumstances either result in that the maximally possible filtration yield cannot be utilized, or that the required outlay for monitoring and regulating the installation becomes very large.
The invention is therefore based on the object of making possible the maximally possible filtration yield, based on the installation, along with a large proportion of solids of the product, without endangering the safety of the operation because of overloads.
In accordance with the invention, this object is attained in connection with a method of the type mentioned at the outset in that the strength of the operating current of the electric motor is measured and, by setting the conveying flow of the pump by means of the adjustment device, is set to at least one predetermined value.
Preferably the method is executed in such a way that the predetermined value of the operating current of the electric motor is a maximally permissible nominal value of the current consumption. In the process, the value of the operating current of the electric motor as the regulating variable is adjusted by means of a regulator to the nominal value through a comparison with the preselected value as the nominal value via the device for setting the conveying flow of the pump as the control system in a control loop.
A flow control valve, which is connected downstream of the pump in the inflow conduit for the product, or a device for setting the frequency of the operating current, and therefore the pump rpm, associated with the current supply of the electric motor, are used as the device for setting the conveying flow of the pump. It is possible in this case to additionally control the inflow pressure into the filtration module by means of a flow control device in the module outlet for the retentate.
Further variations of the method, as well as of a cross-flow filtration installation for executing it, are characterized in the claims.
In comparison with known installations, the method in accordance with the invention also offers the additional advantage in connection with required module cleaning outside of the filtration operation that, because of the safely possible maximum product flow-through, improved cleaning of the modules by means of the flow-through method is possible.